The present embodiments relate to recording a magnetic resonance data set relating to an examination region that is moved at least partly and periodically.
Examination regions that move lead to artifacts in magnetic resonance images and spectra due to phase errors. Allowing for movements in the examination region is therefore known.
The term “periodic movements” in the context of magnetic resonance examinations usually denotes movements caused by the heartbeat and/or breathing. With these, it is possible to split up a movement cycle into a plurality of movement phases.
A plurality of strategies are possible for minimizing movement artifacts. First, the recording may be based in the movement phases of the movement cycle in which the movement amplitude is low. With cardiac images, the measurement may be made during diastole, for example, and with pulmonary images, during expiration.
Triggered magnetic resonance measurements are known from documents DE 10 2009 061 198 B3, U.S. Pat. No. 9,414,767 B2, and U.S. Pat. No. 7,620,441 B2, for example.
By starting out with a trigger signal, one or a plurality of partial recordings may be initiated in order to generate the part of a data set for each movement phase. With movements of the heart, an ECG signal from the test subject or patient is evaluated and the data recording is triggered as a function of the ECG signal. In the case of cine-recordings, after a trigger signal, in each case one k-space line of an image may be acquired consecutively. In this way, a plurality of images are acquired, one image for one movement section in the cardiac cycle. If the images are played back one after another, the heart may be seen beating.
A plurality of methods are known for determining breathing movements. The movements of the abdomen may be detected by a measuring belt. The belt does not affect the MR measurements, but provides only approximate information about the deformation of the abdomen in the examination region.
Therefore, a further known method is recording navigator echoes. Here, one or a plurality of 2D-slices or 3D-volumes are read in predetermined orientations and, from the differences in otherwise identical recording conditions, it is concluded that translations and rotations have occurred in the region being observed. With this method, movements may be detected very accurately, but the drawback is that the signal in the image region of an image may be partly saturated due to these measurements.
A problem arises with all the sequences that work with saturation. Saturation occurs when working with a fat- or water-suppression module or sequences that take measurements in the steady-state.
If one proceeds as described, a measurement is initiated after the input of the trigger signal, irrespective of the saturation status of the magnetization in the examination region. This is not a problem with cine-images because cine-images always start with a completely unsaturated magnetization due to the inflow-effect in the region of interest. In all other cases, the state of magnetization is basically undefined.
In steady-state sequences, to compensate for relaxation effects due to the interval between the end of the recording and the next trigger signal, a known method is transmitting HF pulses even without data recording in order to sustain the steady-state. These pulses are also known as saturation pulses. If the pulse emission and the trigger signal cross over in an unfavorable manner, the targeted movement phase may not be maintained.